Chlorine dioxide is a strong, but highly selective oxidizer. It has been used in aqueous solution for many decades in various applications including disinfecting drinking water and in other water processing applications. One of its chief benefits is that it does not react with organic materials to form chlorinated hydrocarbons, which are increasingly avoided because of health concerns and regulatory pressure. In fact, chlorine dioxide can be used to destroy organic compounds that form chlorinated hydrocarbons, or to destroy chlorinated hydrocarbons after they have been formed.
Aqueous solutions of chlorine dioxide are also used in large quantities for bleaching paper pulp, where use of the chemical has greatly reduced the formation of chlorinated by-products compared to those formed by prior methods. Solutions of chlorine dioxide have been used extensively for decontamination of bio-contaminated buildings, enclosures, and articles. Chlorine dioxide solutions are also used extensively as a disinfecting wash for poultry, beef, and many types of fruits and vegetables. Because of the instability of known chlorine dioxide solutions, these solutions are produced at or near the point of use, and storage times are limited.
Several suppliers offer a liquid called “Stabilized Chlorine Dioxide”, “Chlorine Dioxide Solution” or similar names. These materials are not chlorine dioxide, but dissolved sodium chlorite. When mixed with acid, they produce chlorine dioxide solutions, but this requires chemical mixing and handling of acid. Opportunities abound for errors in mixing and even when reagents are mixed properly the resulting solution may contain high levels of salt, acid, and other impurities. Moreover, after mixing, the chlorine dioxide solutions have a short shelf life.
Gaseous chlorine dioxide is also becoming an increasingly important disinfectant. The gas has been used for many years to sterilize medical instruments and other medical articles as described in U.S. Pat. No. 4,681,739. Gaseous chlorine dioxide has also been used to decontaminate buildings containing Anthrax spores after the Anthrax attacks of 2001. The gas reportedly has been commonly used for decontamination of buildings infested with mold. It is also being introduced as a decontaminant for bio-safety cabinets and other laboratory enclosures.
Chlorine dioxide can be produced in a variety of ways. Most of the production processes suitable for use at less than a few thousand pounds per day are based on reaction of sodium chlorite with chlorine or acid in aqueous solution. Many of these processes are based on the reaction:2NaClO2+Cl2=>2ClO2+2NaCl  Reaction 1
All technologies where chlorine dioxide is produced in solution, whether produced from Reaction 1 or otherwise, produce chlorine dioxide solutions containing the other products and by-products of the reaction plus unreacted feedstock reagents. Typical contaminants in these products include chlorine, various acids, sodium chlorite, sodium chlorate, and sodium chloride.
In recent years, a new process described in U.S. Pat. No. 5,234,678, has enabled the simple and safe production of high purity chlorine dioxide gas. This process involves the reaction of a solid granular sodium chlorite with dilute chlorine gas according to Reaction 1. Unlike the liquid phase production methods, the product resulting from this process does not contain significant quantities of sodium chlorite, sodium chlorate, or substantial quantities of sodium chloride, since these materials do not form gases to any appreciable extent. Tests by an independent lab have shown that the chlorine dioxide gas produced from this process can be over 99.95% pure.
The use of highly pure chlorine dioxide gas as an oxidizer and disinfectant has been limited because chlorine dioxide is unstable in gas phase, and has been thought to have limited stability in aqueous solution. The Handbook of Chlorination and Alternative Disinfectants—4th Edition—George Clifford White, states that “aqueous solutions of chlorine dioxide are subject to photolytic decomposition, the extent of which is a function both of time and of the intensity of the ultraviolet component of the light source. Aqueous solutions of chlorine dioxide are known to retain their strength for longer periods of time if kept cool and properly stored in the dark.” For many applications, however, refrigeration is expensive or impractical, and even with refrigeration the shelf life of chlorine dioxide produced in traditional ways is relatively short. For these reasons, most chlorine dioxide applications currently require generation of the chemical at, or near, the point of use. The literature abounds with references stating that unrefrigerated chlorine dioxide cannot be shipped or stored.
The use of chlorine dioxide solutions has been limited because chlorine dioxide concentration must be kept low for safety reasons. Chlorine dioxide gas above such solutions can decompose spontaneously and exothermically if it reaches elevated concentrations. OSHA lists the safe limit as 10% (76 mm partial pressure) in air at atmospheric pressure. Other expert sources identify the limit as 16% (120 mm partial pressure) or even higher. At a partial pressure of 150 mm and higher, a spontaneous decomposition is quite mild and characterized as a “puff”. At still higher concentrations, the decompositions become explosive, and at partial pressures of 225-300 mm or higher, explosions can be quite violent. The presence of water vapor elevates the concentration at which decompositions occur. FIG. 1 shows the vapor pressure of chlorine dioxide gas above aqueous solutions of the gas as a function of temperature and concentration. As with aqueous solutions of most gases, the solubility of chlorine dioxide decreases as temperature increases—i.e. for a given concentration of dissolved gas, the partial pressure of the gas above the solution at equilibrium is a positive function of temperature.
Even if the concentration of a solution is in a stable range, the shipment and storage of chlorine dioxide solutions must be done with care. It is commonly thought that chlorine dioxide cannot be shipped or stored. Thus, methods are needed for safely shipping and storing chlorine dioxide solutions.
For economic and logistical reasons, it is desirable to ship the most concentrated solutions that can safely be shipped. However, solutions packaged at low temperature under safe conditions might warm up and produce dangerous gas-phase concentrations. For example, solutions packaged at 5° C. and 15 g/L would have a headspace gas concentration of about 11 kPa (84 mm Hg partial pressure), which would be safe. If that same solution warmed up to 20° C., the headspace concentration would reach 20 kPa, which is near the region of spontaneous decomposition. If that solution warmed further to 60° C., the gas phase concentration could become quite dangerous. Solutions having a concentration below 3000 ppm by weight chlorine dioxide in water are regarded as safe for shipment in temperate climates. The gas in the head space above these liquids might reach 110-115 mm Hg if the temperature of the liquid reached 60° C. Solutions up to 2500 ppm could safely be allowed to reach 71° C., which is as high as temperatures are likely to reach in North America or Europe, even in unventilated enclosures in the sun. The use of ventilated warehouses and trucks, could permit still higher concentrations to be used. If the containers of solution could be reliably cooled, even without refrigeration, much higher concentrations are feasible. The extent to which water vapor elevates the safe concentration remains to be tested, but the fact that the gas in the head space of such containers will be saturated with water vapor provides an extra margin of safety.